"obabel ... --gen2D" - Calculated 2D Coordinates

Cheminformatics Tutorials - Herong's Tutorial Examples

∟"obabel ... --gen2D" - Calculated 2D Coordinates

This section provides tutorial examples on how to use Open Babel command '--gen2D' option to calculate 2D coordinates of each atom in the given molecule.

"--gen2D" is an "obabel" command option that tells Open Babel to calculate 2D coordinates of each atom in the given molecule. There are 3 possible ways of using the "--gen2D" option.

1. "--gen2D" Implicitly Applied - If the output format like SVG that requires the molecule to be presented a 2D space, and the input data does not contain 2D coordinates, Open Babel will automatically apply the "--gen2D" option and calculate 2D coordinates of each atom in the molecule.

For example, the following "obabel" command converts a benzene molecule from SMILES format to SVG format without "--gen2D" option. 2D coordinates are calculated automatically.

herong$ obabel -:c1ccccc1 -O benzene.svg

2. "--gen2D" Explicitly Suggested - If the output format like SDF that is capable to record 2D coordinates but not required, and the input data does not contain 2D coordinates, Open Babel will not apply the "--gen2D" option and give a warning message.

For example, the following command converts a benzene molecule from SMILES format to SDF format without "--gen2D" option. 2D coordinates are not calculated.

herong$ obabel -:c1ccccc1 -O benzene-without-gen2D.sdf 
==============================
*** Open Babel Warning  in WriteMolecule
  No 2D or 3D coordinates exist. Stereochemical information will be 
  stored using an Open Babel extension. To generate 2D or 3D coordinates 
  instead use --gen2D or --gen3D.
1 molecule converted

herong$ more benzene-without-gen2D.sdf 

 OpenBabel10092109552D

  6  6  0  0  0  0  0  0  0  0999 V2000
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
  1  6  2  0  0  0  0
  1  2  1  0  0  0  0
  2  3  2  0  0  0  0
  3  4  1  0  0  0  0
  4  5  2  0  0  0  0
  5  6  1  0  0  0  0
M  END

herong$ obabel benzene-without-gen2D.sdf -O benzene.svg

Note that benzene-without-gen2D.sdf is still a valid molecule file. You can structure the benzene structure according to the list of atoms and bonds.

If you want to record 2D coordinates in the SDF output file, you can specify the suggested "--gen2D" option.

herong$ obabel -:c1ccccc1 -O benzene-with-gen2D.sdf --gen2D 
1 molecule converted

herong$ more benzene-with-gen2D.sdf 

 OpenBabel10092110102D

  6  6  0  0  0  0  0  0  0  0999 V2000
   -0.8660   -0.5000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
   -1.7321   -0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
   -1.7321    1.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
   -0.8660    1.5000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
   -0.0000    1.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
  1  6  2  0  0  0  0
  1  2  1  0  0  0  0
  2  3  2  0  0  0  0
  3  4  1  0  0  0  0
  4  5  2  0  0  0  0
  5  6  1  0  0  0  0
M  END
$$$$

3. "--gen2D" Used to Recalculate - When input data already contains 2D coordinates, you can still specify the "--gen2D" option to recalculate them.

For example, the following command converts a guanosine molecule from SDF format to SVG format with "--gen2D" option. 2D coordinates are recalculated.

herong$ obabel guanosine-poor.sdf -O guanosine-with-gen2D.svg --gen2D
1 molecule converted

As a comparison, the following command converts the same SDF file to SVG format without "--gen2D" option. The original 2D coordinates are used.

herong$ obabel guanosine-poor.sdf -O guanosine-without-gen2D.svg 
1 molecule converted

The following picture shows the molecule 2D structure with 2D coordinates recalculated by Open Babel on the left, and the 2D structure of the original 2D coordinates on the right.